Trunking radio systems are widely used in dispatch applications today. A basic trunking radio system is described in U.S. Pat. No. 4,012,597 assigned to Motorola, Inc., and more recent examples of trunking radio systems are disclosed in U.S. Pat. Nos. 4,612,415, 4,692,945, and 4,723,246, all assigned to Motorola, Inc. Because these systems are well-known, the operational details of trunking radio will not be described herein. However, the conceptual basis of a trunking radio is illustrated in prior art FIG. 1.
A typical trunking radio system includes a plurality of subscriber stations 1A-1C, a centrally located repeater 3, and a control station 5. While only three subscriber stations are shown in FIG. 1, there will be typically be many times this number in an actual system. The trunking radio system maintains communication between each subscriber station and the control station 5, as well as between individual subscriber stations. Because the number of channels allotted to the system is typically much smaller than the number of subscriber stations, the control station 5 assigns channels for communications on an as-needed basis.
Channel assignment is typically achieved as follows. One of the subscriber stations initiates a communication on the system by transmitting a channel request to the control station 5 on a predetermined control frequency. The request includes that station's ID and talk group, and is known as the Inbound Signalling Word (ISW). (A talk group is a group of stations with a common interest, such as all of the members of a certain repair team, etc.). The control station 5 responds to the subscriber station via the control channel with an Outbound Signalling Word (OSW) which authorizes the subscriber station to operate on an assigned channel. Thus, the control station 5 knows the identity of the subscriber station which initiated the contact, its talk group, and the channel to which the communication is assigned. The controller station then instructs the other subscriber stations within the initiating station's talk group to receive on the assigned frequency. When the communication is complete, the channel is released and becomes available to control station 5 for further assignment. Thus, a response to the initiating subscriber station from one of the other stations in the talk group is likely to be assigned to a different channel than the one to which the original communication was assigned. When a response is initiated, the control station 5 advises all stations within the talk group of the new channel assignment.
As a result of the above-described arrangement, when, for example, fifty or more talk groups are on the trunking radio system (each of which includes a number of individual stations) each channel carries a virtually constant stream of small segments from unrelated conversations. While the subscriber stations can follow the conversation by switching channels in accordance with the control information, it is impossible to follow any one conversation by monitoring any one of the channels.
It is frequently desirable to maintain a log of all transmissions over a dispatch radio system. For example, when the dispatch system is used for emergency response, it is often desirable to retrieve the original dispatch information and the content of conversations between the dispatched units. With a trunking radio system, however, this is very difficult. The reason for the difficulty is that if one were to simply back up the data on each channel, it would be impossible for the reasons explained above to reproduce any one complete conversation from any one of the channels. On the other hand, recording the conversations of each talk group individually is economically unfeasible because it would require a number of receivers and storage devices equal to the number of talk groups. Clearly, it is not practical to have a separate receiver and recorder for each talk group. Thus, prior to the present invention, there was no economically feasible way in which trunking radio systems could be logged for future retrieval of data.